The demand for continued improvement in the efficiency of gas turbine engines has driven the designers of such machines to specify increasingly higher firing temperatures. Nickel and cobalt based superalloys are now commonly used to form components in the combustion gas flow path because of their resistance to the high temperature oxidizing environment. However, even modern superalloys are not capable of surviving long term operation at the firing temperatures of modern gas turbine engines that may exceed 1,400° C. In order to provide additional protection to the metal components in the hottest areas of a gas turbine engine, it is known to coat the metal substrate with a layer of ceramic material to thermally insulate and chemically isolate the substrate from the hot combustion gasses. A widely used material for this application is yttria stabilized zirconia (YSZ), with 8 wt. % Y2O3 (8YSZ) being a common composition.
The thermal insulating properties of ceramic thermal barrier coatings have been the subjects of many design improvements over the years. U.S. Pat. No. 6,025,078 describes the use of zirconia stabilized with both yttria and erbia. The erbia reduces the thermal conductivity of the material when compared to zirconia stabilized by yttria alone. This patent suggests that the material may include between 4-20 wt. % yttria, while it may include between 5-25 wt. % erbia. Specific embodiments are described as having as much as 29 wt. % combined yttria and erbia stabilizer.
It is also important for a ceramic thermal barrier coating to exhibit phase stability over the expected operating range of operating temperatures. A change in phase may be accompanied by a change in volume, leading to the development of stresses within the coating and between the coating and the substrate. A thermal barrier coating having high phase stability is described in U.S. Pat. No. 6,258,467 as having a pyrochlore crystal structure. The pyrochlore structure is described as having several advantages over a conventional fluorite (cubic) structure, including a higher resistance to sintering. The patent teaches that the oxygen defects in a conventional fluorite yttria stabilized zirconia (YSZ) structure are very mobile and can contribute to sintering, whereas in the pyrochlore structure the oxygen defects are ordered and, hence, can be more resistant to sintering. Another pyrochlore material, lanthanum zirconate, is described in U.S. Pat. No. 6,117,560.
U.S. Pat. No. 4,535,033 describes a thermal barrier coating of zirconia partially stabilized with ytterbia. That patent describes a preferred embodiment having 12.4 wt. % ytterbia and including the cubic, monoclinic and tetragonal phases. This patent illustrates that the number of thermal cycles to failure decreases for this material with an increasing stabilizer content, with data supporting this trend being plotted up to about 25 wt. % ytterbia.
U.S. Pat. No. 6,187,453 describes how a coating formed by an EB-PVD process may not have a composition corresponding to the target material used to form the coating. The patent discloses a process for forming a thermal barrier coating material being a homogeneous mixture of yttria and ceria having 5-60 wt. % yttria with the balance being ceria. This patent also teaches that an increased amount of yttria in ceria will enhance the erosion resistance of the material. In contrast, the patent notes that zirconia stabilized with 20 wt. % yttria demonstrates a dramatically increased rate of erosion when compared to YSZ having only 12 wt. % yttria.
U.S. Pat. No. 6,231,998 discloses hexagonal phase zirconium scandate with up to 42 wt. % Sc2O3. That patent suggests that this material will be more resistant to sintering than YSZ because oxygen vacancies in the crystal structure of YSZ promote diffusion of species through the structure, thereby resulting in relatively easy sintering of the YSZ.
U.S. Pat. No. 4,936,745 discloses an abradable ceramic seal material made of zirconia stabilized with 6-8% yttria (6-8YSZ) and having porosity of 20-30%. This abradable material is prepared by co-spraying a mixture of ceramic powder and a fugitive material such as polyester. The polyester is volatized during heat treatment of the YSZ material, thereby leaving voids that provide a desired degree of porosity for applications where the material must be abraded by a rotating component. This material has been used to coat the seal rings of combustion turbines to provide an abradable seal with the adjacent rotating turbine blade tips. However, operational experience with this material has been less than desirable as the firing temperatures of combustion turbines continue to increase above 1,100° C.
There is an ongoing need for thermal barrier coating materials having improved performance properties in high temperature, corrosive environment applications. In particular, there is a need for an abradable coating that may be used for turbine ring segment applications in modem combustion turbines.